1. Field of the Invention
The present invention relates to a method of and an apparatus for air separation and, in particular, to a method of and an apparatus for air separation of the type in which the material air is cooled through heat exchange with return gases by a main heat exchanger arranged in the air separation apparatus, wherein the pressure loss of the return gas lines is reduced as much as possible, whereby the efficiency and stability of the air separating operation are improved and a reduction in running cost is achieved.
2. Description of the Related Art
The air separation method for separating air into nitrogen gas and oxygen gas is used in various fields, such as steelmaking, chemical and electronic industries. Regarding this air separation method, research is being pursued for the purpose of achieving an improvement in terms of separation efficiency, a reduction in running cost, an improvement in operation stability, etc.
Given these goals, FIG. 1 is a flow diagram illustrating an example of a molecular sieve type air separation method developed and an apparatus for executing the method. Material air is transferred by way of an air filter 1, a material air compressor 2, a cooler 3, etc. and turned into air at a desired pressure, temperature and humidity (hereinafter referred to as "the compressed air") before it is led to molecular sieve adsorbers 6. In the example shown, there are provided a pair of molecular sieve adsorbers 6, which are selectively used. In the adsorbers 6, any water, carbon dioxide, hydrocarbon, etc. are almost completely removed from the compressed air by the adsorbing action of zeolite or the like. The compressed air is then led out of the adsorber 6 and transferred through a duct 6a and led to a main heat exchanger 7, where it is cooled down to a temperature around the liquefaction point through heat exchange with return gases described below before it is led to the lower section of a lower column 8a of a rectifying tower 8.
The compressed air thus led to the lower column 8a undergoes rectifying separation as it ascends within the lower column 8a. As a result, a nitrogen-rich liquid (liquid nitrogen) 9 of low boiling point is extracted from the upper section of the lower column 8a while an oxygen-rich liquid 10 of high boiling point is stored in the lower section of the lower column 8a (This process will sometimes be referred to as the "rough rectification process"). Nitrogen-rich gas in the upper section of the lower column is led through a duct 13 to a main condenser 8b, where it is liquefied before it descends through a duct 14 to return to the upper section of the lower column 8a. The nitrogen-rich liquid in the upper section of the lower column 8a is transferred through a duct 15 and led to the top section of an upper column 8c by way of a super cooler 12.
The above-mentioned oxygen-rich liquid 10, on the other hand, is led to the middle section of the upper column 8c by way of a duct 25 and the super cooler 12. Further, from the middle section of the lower column 8a, the liquid nitrogen which is in the middle stage of the rough rectification process is led to the upper section of the upper column 8c by way of the duct 11 and the super cooler 12. In this way, the lower-temperature liquid nitrogen and the oxygen-rich liquid 10 are led from the middle, upper and top sections of the upper column 8c and descend within the upper column 8c. Mass transfer is effected between the gas ascending within the upper column 8c and the lower temperature liquid nitrogen and the oxygen-rich liquid 10, whereby rectification proceeds.
By repeating these processes, nitrogen gas is separated in the top section of the upper column 8c, while liquid oxygen is stored in the lower section of the upper column 8c, oxygen gas being extracted from a position somewhat higher than the surface of this liquid oxygen. These gases are led to the main heat exchanger 7 through ducts 16 and 17, and heat exchange is effected between the compressed air led out from the molecular sieve adsorber 6 and these gases, whereby these gases are obtained, by cooling, as the nitrogen product and the oxygen product.
Part of the compressed air led out from the molecular sieve adsorber 6 is branched off before being led to the main heat exchanger 7. It is pressurized by a pressurizer 5a on the input side of an expansion turbine 5 and then cooled on the high temperature side of the main heat exchanger 7; then, it is extracted halfway and returned to the expansion turbine 5, where it undergoes adiabatic expansion to be thereby further cooled before it is led to the middle section of the upper column 8c. Exhaust nitrogen gas in a roughly separated state is extracted from a position somewhat lower than the upper section of the upper column 8c by way of a duct 20, and transferred from the super cooler 12 as return gas by way of the main heat exchanger 7 for heat exchange utilizing coldness. After that, the exhaust nitrogen gas which has undergone heat exchange is supplied to the adsorber 6 by way of a regenerative heater 29 and utilized for molecular sieve regeneration in the adsorber 6. The surplus exhaust nitrogen gas is supplied through a duct 21 to an evaporation cooler 4 to be used to cool cooling water to be utilized for cooling of the cooler 3 before it is discharged. After the regeneration/heating of the molecular sieve adsorber 6, the above-mentioned exhaust nitrogen gas is supplied to the adsorber 6 after regeneration/heating by the switching between valves V.sub.1 and V.sub.2 to cool it to thereby complete the preparation for switching to the adsorption process. The exhaust nitrogen gas utilized for the regeneration of the molecular sieve adsorber 6 is sequentially discharged to the exterior of the system.
In the main heat exchanger arranged in such an air separating apparatus, a number of units using corrugate fins (plane-type fins, herringbone-type fins, perforate-type fins, louver-type fins, serrate-type fins, etc.) are superimposed one upon the other and mounted as the main heat exchange members for high heat exchange efficiency, and the fluids to be subjected to heat exchange are caused to flow through adjacent units in opposed flows, whereby heat exchange is effected.
As is known in the art, in such an air separating apparatus, the power of the air compressor indicates the power performance of the apparatus. The lower the pressure at the outlet of the air compressor, the lower the power of the air compressor, and the more enhanced the performance of the apparatus as a whole. In view of this, various examinations are being made for the purpose of lowering the pressure at the outlet of the air compressor to thereby enhance the performance of the apparatus.
In the main heat exchanger used when executing the conventional air separation method, heat exchange is effected between the return gases (which mainly consist of the oxygen gas product, the nitrogen gas product and the exhaust nitrogen gas) and the compressed air (the material air), and the coldness of the return gases are utilized to cool the compressed air, which means, in the main heat exchanger, four kinds of fluid flow through flow passages defined by heat exchange walls. Thus, in order that heat exchange may be efficiently effected between these four kinds of fluid, a structure as shown, for example, in FIG. 6 (a general perspective view), FIG. 7 (a partially cutaway perspective view with the head portion taken away), and FIG. 8 (a front view showing a heat exchange unit provided with distributors for separation between the fluids), is adopted as the main heat exchanger.
In this main heat exchanger, generally indicated by symbol A, a number of heat exchange units in which corrugate fins are superimposed one upon the other through partitions constituting heat conduction walls are stacked together. The flow paths for the fluids are separated by distributors provided on the input and output sides of the heat exchange section. That is, as shown in FIGS. 8(A) through 8(D), four types of heat exchange units A.sub.1 through A.sub.4 are used, in which the flow paths on the input and output sides are varied by means of the distributors in accordance with the four kinds of fluid between which heat exchange is effected, and these units are superimposed one upon the other so as to be adjacent to each other to thereby constitute the main heat exchanger A. In FIG. 8, symbol HE indicates the heat exchange section; and symbols Da, Db, Dc and Dd and numerals D.sub.1, D.sub.2, D.sub.3 and D.sub.4 indicate distributors. In these distributors, the outlet and inlet sections are varied. For example, the heat exchange units A.sub.1, A.sub.2, and A.sub.4 are used as the passages for nitrogen gas, the oxygen gas and the exhaust nitrogen gas, and the heat exchange unit A.sub.3 is used as the passage for the compressed air, the heat exchange units A.sub.1, A.sub.2, and A.sub.4 being stacked together, with the heat exchange unit A.sub.3, which constitutes the passage for the compressed air, being placed therebetween, whereby an assembly as shown in FIGS. 6 and 7 is obtained to form the main heat exchanger A, with headers H being mounted to the inlet and outlet for each fluid.
In this conventional main heat exchanger, however, the flow direction of the fluid is changed in the output side distributor portion of each heat exchange unit, and each fluid flows in a condition in which the passage is narrower in the distributor portion than in the heat exchange section HE, so that the generation of a great pressure loss is inevitable. When such a pressure loss is generated in the passages for the return gases, in particular, in the passage for the exhaust nitrogen gas or that for the nitrogen gas, which gas has a great flow rate, the operating pressure of the air separating apparatus as a whole is adversely affected to a marked degree for the reason stated below, and the power of the material air compressor must be enhanced to a considerable degree.